This invention relates generally to multi-electron beam color cathode ray tubes (CRTs) and is particularly directed to a color index CRT wherein plural electron beams are formed in groups prior to incidence upon the CRT""s display screen.
One type of CRT which does not incorporate a color selection electrode, or shadow mask, employs a large number of narrow substantially parallel phosphor stripes which are arranged in groups of three, with each stripe typically emitting one of the primary colors of red, green or blue. The phosphor stripes may be arranged either vertically or horizontally on the inner surface of the CRT""s display screen. Black inoperative stripes are typically disposed intermediate adjacent color emitting stripes. Multiple index stripes are typically disposed on the display screen""s inner surface for feedback and determining the position of the electron beam. In the case of vertically oriented phosphor stripes, the horizontal scan of the electron beam requires rapid turning on and turning off of the electron beam at the right instant and at a very high frequency, typically on the order of 10 megahertz. In the case of horizontally aligned phosphor stripes, precise x-axis positioning of the electron beam on the phosphor stripes is required. The present invention relates to the latter case of horizontally aligned, vertically spaced phosphor stripes disposed on the inner surface of the CRT""s display screen.
The path of future development in both the shadow mask type and the index type of CRT is in the direction of high definition television (HDTV) displays. Regardless of the CRT configuration, a HDTV display requires a higher frequency magnetic deflection yoke for increased electron beam scan rates and high video image resolution and brightness. Unfortunately, these two operating criteria are inter-related such that improvement in one performance parameter generally comes at the expense of the other.
Increasing the scan frequency of the CRT""s magnetic deflection yoke requires higher deflection input power to the yoke as well as a more expensive yoke assembly. To provide acceptable brightness and resolution in a large 16:9 color CRT, higher beam current and improved video image resolution are required. These enhancements typically require a larger CRT envelope neck size to accommodate a larger electron gun. Increasing the size of the CRT envelope is contrary to current trends which seek to reduce the non-display screen portions of the CRT. One approach to providing acceptable image brightness involving the use of higher beam currents employs a dispenser cathode which affords high electron emission densities. However, the use of a dispenser cathode substantially increases the cost of the cathode. While some of the aforementioned approaches have been adopted in HDTV CRT""s, the increased cost and complexity of the resulting CRT reduces its commercial competitiveness relative to other HDTV display technologies such as liquid crystal displays (LCDs), plasma display panels (PDPs), etc.
Video image brightness is also a concern in projection television receivers. A conventional electrostatic focusing electron gun cannot meet both the beam spot size (resolution) and brightness operating criteria because of the large size of a projection television receiver display. A combined electro- static and magnetic focusing arrangement is typically employed in a HDTV system, which increases the complexity and cost over that of a conventional electron gun and deflection yoke system. In addition, in a high resolution electron gun due to a high video drive frequency, the capacitance of the cathode has to be reduced to 2 pf, or less, which requires a specialized design of increased cost.
The present invention addresses the aforementioned limitations of the prior art by providing a multi-beam group electron gun for beam index CRTs which employs two or more groups of horizontally spaced, vertically aligned electron beams, where each electron beam in a group scans a respective color producing horizontal phosphor stripe. This allows each phosphor stripe to be impinged upon by two horizontally spaced electron beams during each horizontal scan of the CRT""s display screen. A video time delay is used to permit the video information written by the horizontally spaced, vertically aligned electron beam groups to be correlated correctly.
Accordingly, it is an object of the present invention to provide a color index CRT with an electron gun having grouped electron beams, with each group of electron beams providing a portion of a video image on the CRT""s display screen.
It is another object of the present invention to increase the number of video image forming electron beams in a color CRT of the index-type to allow for a reduction in the peak current in each beam without sacrificing video image brightness.
Yet another object of the present invention is to relax magnetic deflection yoke and cathode emission requirements in a color index CRT while maintaining high electron beam spot resolution without increasing CRT neck size or deflection power requirements.
A further object of the present invention is to store received color video information for subsequent recall and display after a predetermined time period on a portion of a CRT""s display screen adjacent to where real time video information is being displayed for the purpose of increasing the portion of the video image displayed with each horizontal scan of the CRT screen.
A still further object of the present invention is to increase by a factor of two the brightness of a video image in a beam index color CRT without increasing electron beam current by doubling the number of electron beams in the CRT.
It is yet another object of the present invention to reduce electron beam spot size in a beam index color CRT for improved video image resolution.
This invention contemplates an electron gun for a color index cathode ray tube (CRT) having a display screen with a plurality of horizontally aligned, vertically spaced phosphor stripes, wherein a video image is formed by sweeping a plurality of electron beams over the phosphor stripes in a raster-like manner, wherein each electron beam provides one of the three primary colors of red, green or blue of the video image, the electron gun comprising: a cathode for providing energetic electrons; a beam forming region (BFR) disposed adjacent the cathode and including first and second spaced, charged grids, wherein each of the grids includes first and second vertically aligned, grouped arrays of apertures for forming the electron beams into a first leading and a second trailing group of electron beams as said electron beams are swept over the display screen, and wherein the first leading and the second trailing groups of electron beams are horizontally spaced from one another, with the electron beams in each group in vertical alignment and directed onto a respective phosphor stripe for providing one of the primary colors; a lens disposed intermediate the BFR and the CRT""s display screen for focusing the electron beams on the display screen; and a plurality of video signal sources coupled to either the cathode or to one of the grids in the BFR for providing color video signals thereto in modulating each of the electron beams in accordance with the color video signals; and a circuit for delaying the video signals displayed by the first leading group of electron beams relative to the video signals displayed by the second trailing group of electron beams in synchronizing the display of those portions of a video image formed by the first leading and second trailing groups of electron beams.